The present invention pertains to the pretreatment of crosslinkable polymers and copolymers with boric acid, boric acid anhydride, or mixtures thereof prior to crosslinking using free radical initiators in the presence of oxygen to form crosslinked polymers with non-sticky surfaces.
It is known from the technical literature that the crosslinking of copolymers, especially of thick and complicated profiles, may be carried out with sulfur using the UHF-hot air method or the shear head-hot air method. It is also known that crosslinking with free radical initiators, such as organic peroxides, in the presence of oxygen leads to incompletely crosslinked and sticky surfaces of the parts (see, for example, Farbenfabriken Bayer, Notice of Mar. 2, 1962, Levapren 450 (ethyl vinyl acetate), Section 7.1.1 to 7.1.5, and notice sheet of Dec. 1, 1965, Urepan (urethane rubber), p. 15, first section, and Chemische Werke Huls, May 1976, Third Edition, Buna AP (EPM/EPDM), insert section "Instructions for processors", p.4).
The stickiness especially occurs in the case of copolymers with high filler contents and extended with oil, such as ethylene-propylene copolymers (EPM), ethylene-propylene diene monomer copolymers (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), natural rubber ((cis-1,4-polyisoprene) (NR), isoprene rubber (IR), ethylene-vinyl acetate copolymers (EVA), acrylonitrile-butadiene-styrene copolymers (ABS), styrene-isoprene-styrene copolymers (SIS), chloroprene rubber (CR), chlorinated polyethylene (DM) and polymethane rubber. In order to obtain the better quality of polymers crosslinked with free radical initiators, crosslinking was carried out with organic peroxide initiators in a molten salt bath, known as the "liquid curing method" (LCM), in a steam tube or in other closed systems. A further possibility is described in German Preliminiary Published Application No. 2,936,906, which describes the treatment of the surface with solutions of metal soaps, such as cobalt octoate, cerium octoate, iron octoate, etc.